Method and device for modifying the polarization state of light

ABSTRACT

A magnetic, single-axis crystal is used to modify the polarization state of light, whereby light passes through pre-determined areas of the crystal. To change the polarization state of the light, a magnetic field pulse is applied to the crystal with a magnetic field amplitude, at which the crystal no longer remains in the single-domain state at the end of the pulse, but returns to a defined multi-domain state that is determined by the direction of the applied magnetic field, thus achieving large usable apertures of the switching element and extremely short change periods. According to the invention, energy is only required for the change operation and not for maintaining a specific state.

OBJECT OF THE INVENTION

The invention relates to a method for modifying the polarization stateof light with a magnetic, single-axial crystal that changes to asingle-domain state under the influence of an outer magnetic field pulsewhereby light passes through the predetermined areas of the crystal, aswell as a device to carry out such a method. Objects of the inventionare methods and devices for modifying the polarization of light beams asthey are employed in optical communications systems, informationprocessing, displays etc. based on modifying the direction, theintensity, and the like, of these light beams.

BRIEF DESCRIPTION OF THE STATE-OF-THE-ART

Many types of optical switching elements have been developed up to nowincluding micro-electric mechanical systems (MEMS), acoustic-optical,liquid crystalline, electronic switchable Bragg gratings, bubble jets,thermo-optical, inter-ferrometric, thermo-capillary,electro-holographic, and magneto-optical systems. MEMS are mostly usedat the present. An important advantage of MEMS is the fact that theybelong to the so-called “latching systems”, which means that they havenon-energy stable switching states and need energy only for theswitching operation.

However, their switching time is rather lengthy: approximately 1 ms.Electro-optical systems have relatively much shorter switching times;for example, the switching time of the new electro-holographic switchingelements is only approximately 10 ns. Nevertheless, these switchingtimes need a permanent energy supply at least in one state. Besides, theinsertion loss of electro-holographic switching elements is rather high:namely about 4–5 dB.

With magneto-optical systems there is created the possibility to combinea short switching time and a low insertion loss with the so-calledlatching function (see above). A multi-stable polarization rotator isdescribed in the invention according to the Austrian patent No. 408.700.Stable states are guaranteed with this rotator through inhomogeneitieson the surface of orthoferrite platelets which hold the domain walls(DW) in predetermined positions. Transition between these stable statesoccurs through displacement of the domain walls between these positionsand it takes place without the creation of new domains. The timerequired for these transitions is approximately 100 ns, which means thatthey are faster by several thousand-fold than the ones for other opticalswitching elements of the latching kind. However, the aperture of theswitching element is considerably restricted. The amplitude of themagnetic field of the driver is rather low and this is the cause thatdomain walls can move only a relatively short distance.

It is the object of the invention to reduce the restrictions of theaperture of the switching element.

This is achieved according to the present invention in that a magneticfield pulse is applied to the crystal with a magnetic field amplitude(H) at which the crystal no longer remains in the single-domain state atthe end of the pulse, but returns to a defined multi-domain statedetermined by the direction of the applied magnetic field. Thus, theaperture of the switching element is enlarged by the use of magneticfield pulses of greater amplitude. The aperture is defined therebythrough the zone that is biased by alternating magnetic pulses. Thiszone represents in the present invention the domain structure occurringafter switching the magnetic pulses off. Relatively large domains appearin orthoferrites whereby correspondingly large apertures of theswitching element can be achieved.

Orthoferrites have a right-angled hysteresis function. The coerciveforce of the orthoferrites is rather high—it is a few kilo-Oersted(kOe). A large energy output is required for the necessary creation oflarge magnetic fields to overcome the coercive force (this factor is ofspecial importance in the construction of densely-packed switchmatrixes) and it can also result in increased inductivity of the system,which lengthens the switching time. Inhomogeneities are used on thecrystal surface to decrease the required intensity of the driver fieldwhereby said inhomogeneities fix the domain walls in the predeterminedpositions. If the distance between the inhomogeneities is short or whenthin orthoferrite platelets are used, then the domain walls move fromone inhomogeneity to the other. This thickness is ≈100 micrometer usedin polarization rotation in the visible or nearly visible infraredspectrum range. It has been found that there is another situation with athicker pattern, namely in case of yttrium orthoferrite crystals of a≧1.2 mm thickness, which are used for a 45° polarization rotation at thewavelength of ≧1.3 micrometer. The use of the magnetic fields on thesecrystals, which are rather strong to change magnetization in the largeareas, causes now the creation of new domains and their expansion,collapse of domain with unfavorable magnetization direction, andmagnetization of the crystal as a result thereof. Should the amplitudeof the magnetic field pulse be rather high (a few kOe), then the crystalremains in a mono-domain state after the end of this pulse and changesof the magnetization direction require again the use of pulses withequal or even higher amplitudes.

If, however, the amplitude H of the pulses is not very high and is justenough to achieve saturation magnetization of the crystal (H=H_(s)),then the crystal returns again to the multi-domain state at the end ofthe pulse (nuclei of the oppositely magnetized domains are, in fact, nottotally suppressed and at the end of the pulse they grow again into newdomains).

Additional characteristics and advantages of the invention method andthe corresponding device are described in more detail in the followingwith the aid of Table 1 and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

Table 1 shows the direction of the magnetization in two crystal areasduring and after, clock-wise and counter clock-wise, pulse applications.

FIGS. 1 a, 1 b, and 1 c show the magnetizations in domains of anorthoferrite crystal during a pulse.

FIG. 2 shows inhomogeneities in the side of a rotator.

In some cases, the direction of magnetization is changed to the oppositein specific crystal areas after the use of the pulses (H≦H_(s)): Anorthoferrite crystal is now viewed which is sectioned perpendicular tothe optical axis. The domain walls in such a crystal are orientedperpendicular to the direction of the crystallographic a-axis (see FIG.1). The magnetization is positive in the upper and lower domains andnegative in the center domain (FIG. 1 a). A magnetic field pulse ofnegative polarity influences now the crystal. The crystal is magnetizedup to the single-domain state when the amplitude of the crystal isapproximately H_(s) (see FIG. 1 b). The crystal subdivides in thedomains at the end of the pulse (see FIG. 1 c). The coupling forces arerather high in the lower and upper area of the crystal and the directionof magnetization remains the same as during the pulse. However, thedirection of magnetization becomes negative in the center area wherecoupling forces are weaker. Inhomgeneities can be used again for thestabilization of the domains as they are described in the invention No.408.700.

If light beams are now guided into different crystal areas, then thepolarizations of the various beams are changed dependent on the magneticdriver field and the position of the beams. In the example in Table 1,the polarization of the beams passing through area 1 are characterizedwith “+” (which means that the polarization direction has rotatedclockwise), and the polarization of the beams passing through areas 2are characterized with “−” (the polarization direction has rotatedcounter-clockwise). Should a magnetic field pulse of negative polaritybe applied, then the polarization of the two beams would be “minus”during the pulse. The polarization of beams 1 and 2 wouldcorrespondingly be “−” (for 1) and “+” (for 2) at the end of the pulse.The application of a magnetic field pulse of positive polarity leads tothe new distribution “+” and “+” and at the end of this pulse there iscreated again the state “+” and “−”. A desired polarization distributionor polarization combination can be achieved through the selection ofpolarity and the chosen duration of pulses.

In the invention according to the Austrian patent No. 408.700,inhomogeneities (i.e. cracks or scratches) on the crystal surface,through which light beams pass, are used to fix the domain walls. Theseinhomogeneities on the surface cause a light dispersion especially inthe employment of such crystals in attenuators.

Deviating from the arrangement according to the Austrian patent No.408.700, inhomogeneities (such as scratches) are applied to the side orsides of the crystal. FIG. 2 shows such inhomogeneities in the form ofcracks or scratches on the side of a rotator. The direction of thecracks or scratches are perpendicular to the crystallographic a-axis andparallel to the planes of the domain walls.

Relatively thin platelets are to be used to guarantee continuousmovement of the domain walls across large distances (“relative thin”means platelets of a few hundred micrometers in thickness). In a verywide area of the magnetic field amplitude there exists the influence ofthe magnetic field onto these platelets in the expansion of the presentdomains with an appropriate polarity and not in the creation of newdomains. The inhomogeneities hold the domain wall in the desiredpositions whereby a multi-stable operation of the rotator is madepossible. Stacks of a few such platelets can be used to construct arotator having the desired thickness.

The inhomogeneities can furthermore be combined with the source ofpermanent magnetic fields whereby said inhomogeneities fix the domainwalls. It is proposed in the Austrian patent No. 408.700 to use theinhomogenous magnetic field of a pair of magnets. However, the use oftwo magnets increases the dimensions of the elements or the systems.

Only one permanent magnet is now used according to the invention. Thispermanent magnet maintains magnetization of the adjacent part of therotator; the position of the border of these domains (its domain walls)changes under the influence of the magnetic field pulse and saidposition can be fixed through inhomogeneities as mentioned above.

((German terms shown in the drawing)) Tabelle 1 = Table 1 Bereich = areaPositives Puls = positive pulse Negatives Puls = negative pulse Währenddes Pulses = during pulse Nach dem Puls = after pulse

1. A method for modifying the polarization state of light with amagnetic, single-axial crystal that changes to a single-domain stateunder the influence of an outer magnetic field pulse whereby lightpasses through predetermined areas of the crystal, characterized in thata magnetic field pulse is applied to the crystal with a magnetic fieldamplitude (H) at which the crystal no longer remains in thesingle-domain state at the end of the pulse, but returns to a definedmulti-domain state determined by the direction of the applied magneticfield.
 2. A method according to claim 1, whereby domain walls are heldin predetermined positions through inhomogeneities created in thecrystal.
 3. A method according to claim 1, whereby light beams areguided through areas of the crystal which remain magnetized with thesame polarity sign as the outer magnetic field pulse after switching-offthe outer magnetic field pulse.
 4. A method according to claim 1,whereby light beams are guided through areas of the crystal which remainmagnetized with the opposite polarity sign after switching-off the outermagnetic field pulse.
 5. A method according to claim 1, whereby lightbeams are guided through areas of the crystal which are magnetized withthe same polarity sign during influence of the outer magnetic fieldpulse, and which are magnetized with the opposite polarity sign afterswitching-off the outer magnetic field pulse.
 6. A device for modifyingthe polarization state of light beams according to the method in claim1, having a magneto-optical rotator made of a magnetic, single-axiscrystal, which has inhomogeneities that fix the domains in predeterminedpositions, wherein said inhomogeneities are disposed on the sides of thecrystal.